It is really about whether it is an inducer or inhibitor to liver enzyme - grapefruit is an inhibitor.
Gleevec is metabolized (broken down) by the liver. The enzyme most responsible for this is called CYP3A4 (one part of the "cytochrome P450" family of liver enzymes). The next most important enzyme for metabolizing Gleevec is CYP2D6.
Drugs that inhibit the activity/production of these two enzymes (but especially CYP3A4) slow down the metabolism of Gleevec. This results in a higher concentration of Gleevec in the blood. This could result in more side effects from Gleevec; similar to taking a higher dose of Gleevec. This would be more of a concern for patients already taking a high dose of Gleevec especially if they were already having significant Gleevec side effects.
For patients on a lower dose of Gleevec that were having few or no side effects, then taking a drug that might "inhibit" CYP3A4 or CYP2D6 would probably have less effects, and perhaps not even be noticed.
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